In optical networks that use dense wavelength division multiplexing (DWDM), multiple wavelengths of light are used to support multiple communications channels on a single fibre. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometre fibre-optic links. A typical amplifier may include EDFA components that are pumped with diode lasers. The EDFA stages increase the strength of the optical signals being transmitted over the fibre-optic links. It is known for such amplifiers to include automatic gain control (AGC) for providing a constant gain regardless of variation in the input power and the wavelength composition of the amplifier input.
The gain of EDFA stages depends on the inversion level of erbium ions in the fibre. If, for example, the inversion level of a given stage is high, the gain of the stage will be high. If the inversion level of a stage is low, the gain of the stage will be low. Unless control electronics are used to maintain a steady inversion level under various operating conditions, the gain of EDFA stages will be subject to unacceptable transients. Gain transients in an amplifier may cause fluctuations in the power of the output signals from the amplifier. If the output signals are too weak, it may not be possible to detect the signals. If the output signals are too strong, the signals may be subject to nonlinear optical effects in the fibre.
The graph of FIG. 1 illustrates the variation of the input and output power values with gain for different pump powers of an ideal optical amplifier. A key is shown of the different curves denoting variation of the output pump power Pout with the input pump power Pin for pump powers of 80 mW, 124 mW and 155 mW respectively, and of the different curves denoting variation of the input pump power Pin with linear gain for pump powers of 80 mW, 124 mW, and 155 mW respectively. This shows that the gain profile of the amplifier varies with the pump drive conditions, and therefore that external conditions, such as the adding or dropping of channels, can result in undesirable power transients.
FIG. 2 shows a known AGC for a fixed gain EDFA in which an input power detector 2 is provided in the form of a tap-off coupler for monitoring the power Pin of an input signal to the EDF 1, and an output power detector 3 is provided in the form of a tap-off coupler for monitoring the power Pout of an output signal from the EDF 1. The output signal from the output power detector 3 is supplied to an analogue-to-digital converter (ADC) 4 which in turn supplies an output signal Pmeas indicative of the measured power output signal to one input of a comparator 5. A signal Pset indicative of the target power output is supplied to another input of the comparator 5, this signal being calculated by adding together in an adder 6 a signal Fase from an ASE compensator 7 indicative of a compensating factor for compensating for the effect of ASE noise in the amplifier and a signal G.Pin supplied by a multiplier 9 which is the product of a signal G indicative of the target gain from a gain setter 8 and a signal Pin supplied by an ADC 10 connected to the output of the input power detector 2.
The error signal e(t) that is the difference between the two input signals supplied to the comparator 5 is supplied to a PI or PID regulator 11 which in turn supplies a feed back signal FB which is a function of the error signal e(t), by way of a current limiter 12 for clipping the maximum current Imax , as a pump drive signal to the pump driver 14. Such a known AGC suffers from the main limitation in use that it has too slow a response time with the result that, for example, a required adjustment of the pump drive current occurs a significant length of time after the triggering increase in the output power and undesirable output power transients are produced.
FIG. 3 shows an alternative known AGC for a fixed gain EDFA in which an input power detector 2 is again provided in the form of a tap-off coupler for monitoring the power Pin of an input signal to the EDF 1, and the output of the input power detector 2 is connected to an ADC 10. The output signal Pin from the ADC 10 is applied to one input of a multiplier 20 to another input of which a target gain signal m from a gain setter 21 is applied. The output signal from the multiplier 20 is applied to one input of an adder 22 to another input of which a constant offset signal c from an offset setter 23 is applied. This provides a feed forward signal FF=m.Pin+c that is supplied, by way of a current limiter 12 for clipping the maximum current Imax , as a pump drive signal to the pump driver 14. Such a known AGC suffers from the main limitation in use that it is inaccurate, although it has a much quicker response time than the AGC with feed back control described above with reference to FIG. 2. In particular such a known AGC does not provide temperature or aging compensation with the result that undesirable gain and transient control errors are produced.
U.S. Pat. No. 6,414,788 discloses an AGC for a fixed gain EDFA that combines the feed back control described above with reference to FIG. 2 with the feed forward control described above with reference to FIG. 3. This enables the advantage of the rapid response time provided by the feed forward control of FIG. 3 to be combined with the advantage of greater accuracy of the feed back control of FIG. 2. However such an AGC is limited generally to a fixed gain condition. In such an AGC, the coefficient of the control loop is set to a fixed value determined either by the design or empirically, and would not be changed during a gain set point change.
U.S. Pat. No. 6,975,449 discloses an AGC based on adaptive feed back that dynamically adjusts at least one of the control coefficients in relation to the measured output power. In this case a set point gain change as demanded by the variable gain amplifier (VGA) will change the measured power condition and change the control coefficients accordingly. However the lack of a suitable feed forward scheme limits the speed of response in this approach.
U.S. Pat. No. 6,522,460 discloses a power controller that effectively combines the feed back control described above with reference to FIG. 2 with the feed forward control described above with reference to FIG. 3. However such a device is incapable of being used as an AGC as control is effected on the basis of a target power output Pset rather than on the basis of a target gain.
It is an object of the present invention to provide a variable gain optical amplifier in which gain transients are accurately and rapidly controlled for a range of gain conditions.